Historically in the United States, metropolitan mass transit systems have depended to a great extent upon free wheel transport devices such as busses to carry the main passenger loads because of the great expense inherent in the construction of rail systems of the subway type. At one time fixed wheel tram or trolley cars were used which were electrically powered using overhead power collection devices. In the recent past an increasing emphasis has been placed upon environment and the appearance of the skylines in most metropolitan areas and because of this many overhead systems have been dismantled and replaced with busses. Now, with the increasing interest in "clean air" and the reduction of air pollutants found in metropolitan areas by reducing the allowable amount of combustion products which may be emitted from the internal combustion engine the interest in efficient electrically powered mass transit vehicles has begun to grow. Coupled with this renewed interest has been the interest in electrical powered mass transit systems which has been created by the current shortages of petroleum products.
In the past, underground power collection systems for street railways have been of the plow conduit type. In the plow conduit type power collection system a "plow", the electrical contact device carried by a railway car, is inserted into a conduit which is buried in the street, where it makes contact with a second electrical conductor whereby electrical energy is transmitted to the driving mechanism of the rail car. Prior art plows have either been of a rolling or sliding type and consequently speeds attainable by the mass transit car were low because of friction and other mechanical factors inherent in the design of a plow-conduit type of power collection system.
Generally the prior art conduit is embedded in the street and anchored to a concrete base with anchor bolts and is generally covered by a rigid slotted resilient plate which in some cases had a flexible sealing material positioned within the slot to impede the entry of dirt and mositure into the rail area and to reduce the danger of electrical shock to pedestrians and passengers of free wheel vehicles. Because of its construction it has been necessary to locate special pits along the course of the prior art conduit to allow for the insertion and removal of the plow from the conduit and to provide means for reversing the direction of the car. Further, special tools have generally been required to maintain the prior art conduit. Generally, prior art conduit has been subject to electrical failures because of moisture, dirt and foreign objects which penetrate into the conduit through the plow slot and despite precautions taken the prior art conduit is inherently dangerous to pedestrians because of the structure of the plow slot and the resilient cover plate.
Despite the many advantages of the steel covered conduit system major problems with their use are:
1. Systems using steel or metal cased power collections are limited to a relatively lower safe operating speed;
2. The rubber plow slot covers rapidly deteriorate because of heat generated by the rapid passage of the plow through the rubber casing; and,
3. The plow slot opening allows water, snow, dirt and foreign objects to penetrate into the area of the conductor thereby causing electrical failure and danger to pedestrians and passengers.